Bias voltage supply



April 16, 1957 J. L. NEAL BIAS VOLTAGE SUPPLY 2 Sheets-Sheet 1 FiledApril 13, 1953 MAIN LOAD B W MU MH L o O a.- T5 NU T L L R m mM a MW ummam 9 a m r 7 5 L 3 l w M r 6 r w 3 4 u 3 2 m 4 m L H 7 "M MU T. N /M WK0 T w 7 an A J v 5 April 16, 1957 J. L. NEAL 2,789,266

BIAS VOLTAGE SUPPLY Filed April 15, 1953 2 Sheets-Sheet 2 M/VENTOR J'QCKL. N E9 L United States Patent BIAS VOLTAGE SUPPLY Jack Laurance Neal,New Haven, Conn. Application April 13, 1953, Serial No. 348,255

9 Claims. (Cl. 321-16) This invention relates to a device for obtaininga negative grid voltage from a plate current supply for electrondischarge tube circuits. More particularly it provides a simplearrangement that will produce negative grid voltages when it is appliedto the full wave rectifiers commonly used in power supplies forelectronic devices.

It is one of the principal objects of this invention to provide asimple, efi'ective device for supplying negative grid bias potential or"relatively constant magnitude which will be suitable for use as fixedgrid bias voltages in thermionic tube circuits.

Another object of this invention is to provide a source of negativeunidirectional potential, or of alternating potential, the magnitude ofwhich can be made to vary with load current drawn from the full waverectifier to which this invention is applied.

Other objects will appear hereinafter.

As is Well known in the art, many applications of electron dischargetubes require the control grids of the tubes to be maintained at anegative potential with respect to the cathodes such that no gridcurrent will flow and no power will be consumed by the grid circuit.Heretofore various circuit arrangements have een employed in applying aconstant negative bias voltage to grids of electron discharge tubeswhereby this desired condition can be maintained. One common method hasbeen the use of a separate source of electrical energy, such as a drycell, but this has been found unsatisfactory because of the limited lifeof such dry cells and the consequent requirement that they beperiodically replaced. Another method involves providing a separaterectifier to supply bias voltages, but the size and expense of suchunits makes them uneconomical for use in most electronic devices. One ofthe most common methods of obtaining grid bias potential, known ascathode biasing, de-

pends on the flow of plate current through a cathode resistor, such thatthe bias potential developed is subject to variations with the signal.These variations in potential can never be completely bypassed, with theresult that distortion is introduced into the output circuit of thetube. In circuits requiring a relatively high value of negative gridbias, cathode biasing also has the disadvantages of appreciably loweringthe cathode-plate potential of the tube to which it is applied.

in the present invention I claim to have developed a device to supplynegative grid bias potentials substantially free of the disadvantagesassociated with the above mentioned methods. Moreover, as will beapparent to anyone skilled in the art, a source of negative potentialsuitable for use as grid bias voltage may also "be used in other circuitarrangements where, as in the case of delay voltages for automatic gaincontrol, a source of negative potential of small magnitude is all thatis required, and the current drawn therefrom is negligible.

In still other applications of thermionic tubes, such as automaticregulator circuits, it is sometimes desirable to apply a negative gridvoltage'whi-ch varies with the total 'ice direct current load that thetube circuit imposes upon the power supply. As an example of this modeof operation, one may refer to voltage regulated power supplies whichutilize an electron discharge tube as a variable load to compensate forfluctuations in the main current drain on the supply. In such cases thenegative voltage on the grid of the variable load tube is made to varywith the main load current. By proper design, the present invention willprovide a source for a ne ative unidirectional voltage suitable for suchapplications.

The invention will be best understood by reference to the accompanyingdrawings wherein like numerals of reference refer to the same orcorresponding parts in the several figures.

Figure 1 is an embodiment of my invention when used to supply fixednegative bias to the grid of an audio amplifier tube;

Figure 2 is a symbolic block diagram showing a preferred embodiment ofmy invention when it is used as a source of control voltage in a voltageregulated power pp y;

Figure 3 is a schematic diagram giving the details of the preferredembodiment shown in Figure 2;

Figure 4 is a symbolic diagram of the equivalent circuit of the highvoltages secondary winding of a transformer and such other parts of afull wave rectifier circuit as are necessary to explain the essence ofmy invention.

in all figures, 1 represents a power transformer with primary winding 2connected to a source of single phase alternating current and a highvoltage secondary winding generally represented at 4 which iscentre-tapped at 7 and t ereby divided into two substantially equalsections 5 and 6 as shown. The outside terminals 8 and 9 of Winding 4are connected to anodes 13 and 14 or" rectifier 15. The rectifiercathode 18 feeds positive plate voltage terminal 2t), the negativeterminal 21 being the centre-tap 7 which is connected to ground asshown. This arrangement is in accordance with the conventional seriestype full wave rectifier circuits in which the rectifying device is inseries with the positive side of the direct current load. This type ofcircuit must be distinguished from the so called shunt connected fullwave rectifiers, to which my invention is not applicable, wherein theload is in parallel with the rectifier. For the sake of completeness,the schematic diagrams 1 and 2 show low voltage secondary winding 16which feeds the rectifier heaters 17 and a smoothing filter (also shownin Figure 2) generally represented at 19 is inserted between therectifier cathode 18 and the positive plate voltage terminal 20. In eachof the figures I have shown a full wave thermionic rectifier: it will beobvious to a person familiar with the art that other types of full wave(or indeed two half-wave) rectifiers such as selenium rectifiers, mightbe used instead of the high vacuum or gas filled types indicated in thedrawings. In all cases, however, the outside terminals 8 and 9 ofwinding 4 are connected to what may conveniently be called the inputelements of the rectifier, the instantaneous polarity of whichdetermines the segment of winding 4 which is to supply the load current,and the main load on the rectifier circuit is connected between theoutput element of the rectifier and the centre tap 7 of winding 4.

Referring now to Figure 4 by which the basic operation of my inventionmay most easily be explained, the high voltage secondary winding 4 isrepresented by its equivalent circuit, segment 5 shown as consisting ofthe series combination of alternating current generator 41 (representingthe elect-romotive force induced in segment 5 by actionof the primary)and the resistive and leakage inductance elements 30 which represent itsinternal impedance. Segment 6 of secondary winding 4 is similarilyrepresented by alternating current generator 42 in series with impedanceelements 31. The outside tenninals 8 and 9 of secondary winding 4 areconnected to anodes 13 and 14 of rectifier 15 as explained above.Cathode 18 is connected to positive direct current output terminal 20and tap 7 is connected to ground and negative output terminal 21, withthe load 46 connected between terminals 20 and 21 as shown. Connectedacross the tenminals 8, 9 of secondary winding 4 is a voltage divider 3consisting of two impedance elements iii and 12 connected to biasvoltage output terminal 35 as shown. As this description proceeds, itwill become evident that this voltage divider, the potential developedthereon at tap 11, and the applications in which this potential can beused are of the essence of my invention.

For the purposes of the discussion which follows immediately, it will beassumed that the voltage divider 3 will provide a direct current path;the use of and efiect of series capacitive units will be discussedlater. Furthermore, the term positive and negative, when used todescribe the potential of :a given point in the circuit, will indicatethe, polarity of that point with respect to tap 7 which, in turn, isconnected to ground as discussed above.

Because of the action of the rectifier in switching the unidirectionalload current from one segment of the winding 4 to the other, such thatthe current is flowing alternately through segment 5 when terminal 8 ispositive and through segment 6 when terminal 9 is positive, there is afluctuation in the absolute (i. e. without regard to pclarity) magnitudeof the peak value of alternating potential of terminals 8 and 9 withrespect to ground. The positive peak in each case is smaller than thenegative peak because of the voltage drop caused by the flow of loadcurrent through the internal impedance of the respective segments ofwinding 4. Thus, to take a specific example, if, :at a given instantterminal 8 has attained its positive peak, terminal 9 will be at itsnegative peak, but the positive potential of 8 will be less than thenegative potential of 9 because the drop occasioned by the internalimpedance of the transformer occurs only in the segment in which theload current is flowing-4. e. segment 5. On the next half cycle thesituation will be reversed so that, with respect to ground, terminal 9is less positive than terminal 8 is negative. This fluctuatmg potentialacross the ends of voltage divider 3 is therefore unbalanced to theextent that the negative peaks are of greater magnitude than thepositive peaks, with the actual difference in the magnitudes dependingupon the internal impedance of the respective segments and the loadcurrent which flows through them.

It will thus be evident that while at any given instant it is possibleto select a point on the voltage divider 3 which will have aninstantaneous potential equal to zero with respect to ground, it is notpossible to select a point which will have an average or unidirectionalpotential equal to zero. Therefore, at any given tap 7 on the voltagedivider 3, it is possible to obtain a potential having a pulsatingunidirectional component that is negative with respect to ground, themagnitude of which depends primarily on the internal impedance of thetransformer (which can be considered constant) and the load current.Moreover, to the extent that taps 7 and 11 are not positioned in theelectrical centres of winding 4 and voltage divider 3 respectively,there will also be present at tap 11 a purely alternating potential withrespect to ground. In general it will be desirable to balance out thispurely alternating component by locating tap 7 as near as possible tothe electrical centre of winding 4 and similarly selecting equal valuesfor impedance elements 10 and 12 so that the potential of tap 11 variesdirectly withthe load current and is unidirectional (albeit pulsating)with respect to ground.

If, contrary to the assumption made above, the impedance elements 10 and12 are capacitors as shown in v Figure 2, the potential at tap 11 willnot be unidirection- :al since the capacitors block out the directcurrent component and allow only the alternating component to pass. Iftap 7 is adjusted so as to be in the electrical centre of winding 4 andthe values of impedances 16 and 12 (in this case capacitors) are madeidentical the potential of tap 11, because of the unbalanced positiveand negative peaks discussed above, will be alternating with respect toground and of a magnitude that varies directly with the load current. Aswill be obvious to any one skilled in the art, this alternating voltagecorresponds to the pulsations in unidirectional voltages that would havebeen produced if a voltage divider providing a direct current path hadbeen used. Indeed this same alternating voltage can be obtained from apurely resistive centre tapped voltage divider by feeding the voltageobtained at the tap through a capacitor, thereby blocking outunidirectional component and allowing only the alternating component topass.

Referring more particularly to Figure 1 in which the voltage dividerconnected across winding 4 consists of two equal resistors 10 and 12,the pulsating, negative voltage thereby obtained at tap 11 is fedthrough the filter generally represented at 22. Because of the very lowcurrent drain, this filter 22 may be of the convenient and inexpensiveresistance capacity type, shown as consisting of series resistor 23 andshunt capacitor 24. At the output terminal of the filter, the smoothedunidirectional negative voltage is applied across potentiometer 32 whichis connected between 35 and 21 ground) :as shown. An audio amplifierstag generally represented at 25, consists of triode tube 26 with plate27 connected to positive plate supply terminal 29 through output loadingdevice 33. The cathode 23 is grounded and the audio frequency signalinput 63 is impressed upon grid 29 as shown. The grid resistor 34 isconnected at its upper end to grid 29 and at its lower end to thevariable tap on potentiometer 32. By adjusting the position of this tapan appropriate value of negative potential will be applied to grid 29through resistor 34, thereby maintaining the grid at a negative biaswith respect to grounded cathode 28. By choosing values of resistors 10and 12 which i are low relative to the input impedance presented byfilter 22 and potentiometer 32, it is possible to reduce the directcurrent voltage drop occasioned by resistors 10 and 12 and therebyobtain a proportionately greater negative voltage at terminal 35.Moreover, because filter 22 has efiectively removed the pulsations inthe negative voltage available at tap 11, the output voltage developedacross potentiometer 32 will be essentially constant and satisfactoryfor use as a source of fixed bias voltage.

Figures 2 and 3 show an embodiment of my invention in which it is usedas a source of control signal in a voltage regulated power supply,Figure 2 being a block symbolic diagram showing the functions of theseveral components schematically represented in Figure 3. In Figure 2,those parts not previously referred to consist of control tube 4%),which automatically compensates for changes in current consumed by mainload 46, and the control circuit itself 36, 37, 38, and 39. As shown inFigure 3, cathode 43 of control tube is grounded and plate 44 isconnected to the positive supply lead from the rectifier, eitherdirectly to cathode 18 (as in the embodiment shown) or alternatively onthe main load side of filter 19. When thus connected, triode 40 servesas a secondary load on the power supply in addition to the main load 46,and the current drawn by this secondary load can be varied by changingthe potential of grid 45. Automatic regulation of the voltage output ofthe power supply is secured by arranging that the voltage drop caused bythe transformer and rectifier is kept at a relatively steady, fixedvalue. This is achieved by keeping the total load current flowing fromrectifier cathode 18 essentially constant, irrespective of changes incurrent drawn by main load 46, or, in other words, reducing the internalimpedance of the power supply, as it appears to. a load con nectedacross its output terminals and 21. To obtain this desired condition ofconstant total load current, it is necessary to arrange thesupplementary load (i. e. triode in such a manner that it will drawnmore current when main load 46 draws less and vice versa. This can berealized by arranging for the negative potential on grid to varydirectly with small changes in total load current.

As discussed previously, when the tap 7 is at the electrical centre ofwinding 4 and the elements 10 and 12 are of equal impedance, thepotential at tap 11 (pulsating and unidirectional when 3 provides adirect current path, and alternating when it includes series capacitiveelements) has a magnitude that bears a direct relationship to the totalload current being drawn from the rectifier cathode. By analysis andexperience I have found that this relationship is substantially linearover that part of the transformer and rectifier characteristics usuallyused in power supply circuits. It is clear, therefore, that themagnitude of this voltage, be it pulsating and unidirectional oralternating, is a direct indication of the load on the rectifier and istherefore useful as a control signal. Indeed, when a relatively highimpedance voltage divider consisting of two equal resistors 10. and 12is used, the pulsating negative potential available at tap 11 can beapplied directly to the grid 45 of control tube 40, and a certain amountof voltage regulation obtained thereby. However, I have found that withsuch an arrangement 'a given change in total load current causes sosmall an increment in control voltage relative to the total controlvoltage available at tap 11 that it is not sufiicient to provide thedesired change in plate current drawn by the control tube 4%. Thisdisadvantage may beovercome by using equal capacitive elements 10 and 12 as shown, thereby securing an alternating signal voltage at tap 11This voltage is then fed into a base clipper arrangement (a device Wellknown in the art) which consists of an amplifier tube so arranged thatonly a given portion of the negative peaks of the signal voltage appliedto its grid will etfect its plate current. Consequently the signaldeveloped across the load in the plate circuit of such a tube will bemore an amplification of changes in signal voltage (since these appearas changes in the peak potential applied to the grid) than it will be ofthe absolute value of signal itself. The base clipper 36 forms part ofthe grid circuit of amplifier 37, and the output of amplifier 3'7, whichis a measure of the changes in signal voltage taken from tap 11, isrectified by control signal rectifier 38, and applied to the grid ofcontrol tube it} through filter 39, as best shown in Figure 2.

Referring now to the details of this arrangement as shown in Figure 3,equal capacitors 10 and 12 are connected in series between terminals 8and 9 of secondary winding 4. The voltage at tap 11 is fed directly tothe grid 43 of the triode section of tube 47 and grid 48 is alsoconnected, by means of grid leak resistor 52, to the variable tap 62 onpotentiometer 53. The lower end of potentiometer 53 is grounded asshown, the upper end being connected. to a source of positive potential(in this case rectifier cathode 18) through a filter consisting ofcapacitor 54 and resistor 55. The cathode of tube 47 is grounded and theplate 49 is connected to one end of the primary 5% of transformer 57,the other end being connected to rectifier cathode 18 through resistor56. Secondary winding 59 of transformer 57 is connected between diodeplate 5 of tube 47 and grid 45 of control tube 29. Resistor as, whichacts as a diode load resistor, is connected between grid 45 and ground.A capacitor 61, hunted across resistor 6%; provides filtering action forthe unidirectional voltage which is developed across resistor 6t andapplied to grid 45. In this particular embodiment I have shown a singletube 47 which includes a triode and diode having a common cathode. It isobvious that these functions can be fulfilled by two separate tubes, or,alternatively, a triode tube may be used for the clipper-amplifiercircuit, and a crystal diode used as a rectifier.

In the absence of signal from tap 11, the relatively high positivepotential at the upper end of potentiometer 53 will cause grid 48 to.assume a positive potential and draw grid current, the magnitudes ofwhich depend upon the characteristics of tube 47, the grid leak resistorand the position of variable tap 62 on potentiometer 53. Because gridleak resistor 52 has a value of several mego'hans, even small amount ofgrid current will cause a considerable voltage drop to be developedacross it, so that grid 43 will attain a potential which is onlyslightly positive. Moreover, it is not possible for signal voltage fromtap 11 to appreciably increase the positive potential of grid because inso doing the grid current would increase, causing a correspondingincrease of potential. drop across grid leak resistor 52 and therebykeeping the grid at approximately the same potential. However, when thesignal voltage reaches a negative value suflicient to overcome theslightly positive potential assumed by the grid, the grid currentceases, the signal voltage gains control of the grid potential, andtherefore, the plate current. Thus it will be seen that depending uponthe position of the variable tap on potentiometer 53, a given portion ofeach negative peak of the signal voltage is amplified and reproduced inthe plate circuit of tube 47. Because a given increment in the signalvoltage from tap 1-1 is greater relative to. a portion of each negativepeak than it is to the amplitude of the full negative half cycle,changes. in input signal are accentuated in the triode plate circuit oftube $7. This signal receives further amplification by audio transformer57 and is rectified, filtered and appears as a unidirectional negativepotential on grid 45 of control tube 49.

From the foregoing it will be evidentthat the negative voltage on grid45 of control tube 40 will. adjust itself automatically to compensatefor changes in the total? load current, the mean value of Which can beadjusted over a narrow range by the position of the variable tap 62 onpotentiometer 53. Consequently, if load 46 suddenly draws less current,the total load current will tend to ecrease, thereby causing the signalvoltage at tap 11 to decrease. This in turn reduces the negative voltageapplied to grid 45 and causes control tube 40 to draw more current andrestore the total load current to approximately its previous value.Moreover, because of the clipper amplifier arrangement, a small changein voltage from tap 11 will cause a relatively larger change in therectified negative voltage applied to grid 45 so that a very high degreeof regulation is possible.

What I claim as my invention is:

1. In a power supply circuit which uses a single series connected fullwave rectifier and a power transformer with a centre tapped secondarywinding whose outside terminals are connected to the input of therectifier, a main load circuit connected in series between the output ofthe rectifier and the centre tap, a source of D. C. potential having apolarity relative to the centre tap opposite to the D. C. potential atthe output of the rectifier, said source being obtained from thejunction of two equal impedance providing a D. C. path connected inseries between the outside terminals of the secondary winding.

2. In a power supply circuit which uses a single series connected fullwave rectifier and a power transformer with a centre-tapped secondarywinding whose outside terminals are connected to the anodes of therectifier; a main load circuit connected between the cathode of therectifier and the centre tap, a source of D. C. potential which isnegative with respect to the centre tap, said source being obtained atthe junction of two equal resistances connected in series between theanodes of the rectifier.

3. In a circuit for supplying plate and grid potentials to electrondischarge tubes; a single series connected full wave rectifier, a powertransformer having a centre tapped secondary winding whose outsideterminals are connected to the anodes of the rectifier, a load circuitconnected between the cathode of the rectifier and the centre tap, twoequal resistances connected in series between the anodes of therectifier, voltage divider means connected between the junction of theresistances and the centre tap, means whereby at least part of thepotential across said voltage divider means is used as grid bias in theelectron discharge tube circuit.

4. In a circuit for supplying plate and grid potentials to electrondischarge tubes: a single series connected full wave rectifier, a powertransformer having a centre tapped secondary Winding whose outsideterminals are connected to the anodes of the recitfier, connection meanswhereby the plate voltage for the electron discharge tubes is obtainedfrom between the cathode of the rectifier and the centre tap, two equalresistances connected in series between the anodes of the rectifier, avoltage divider connected between the junction of the resistances andthe centre tap, and filter and conductor means whereby at least part ofthe voltage developed across said voltage divider is used as grid biasin the electron discharge tube circuit.

5. In a voltage regulater power supply circuit: a single full waverectifier, a power transformer having a centre tapped secondary windingwhose outside terminals are connected to the input of the rectifier, aload connected in series between the output of the rectifier and thecentre tap, two equal impedance elements connected in series between theoutside terminals of the winding and means whereby the potentialdifference between the junction of the impedances and the centre tap isused as a source of control signal for an element in the circuit whichcontrols the output voltage of the supply.

6. In a voltage regulated power supply circuit using a single full waverectifier, a centre tapped transformer winding whose outside terminalsare connected to the input elements of the rectifier, the plate andcathode of a control tube connected across the output of the rectifierin parallel with the load, two equal impedances providing a D. C. pathconnected in series between the input elementsof the rectifier, andmeans whereby at least part of the potential difference between thejunction of the inipedances and the centre tap is applied to a grid ofthe control tube.

7. The circuit as claimed in claim 6 in which the full wave rectifier isa full Wave thermionic rectifier and the input elements are the anodesthereof.

8. In a voltage regulated power supply circuit using a single full waverectifier, a centre tapped transformer winding whose outside terminalsare connected to the anodes of the full wave rectifier, a control tubehaving a plate and cathode connected in parallel with the load betweenthe cathode of the rectifier and the centre tap, two equal capacitorsconnected in series between the anodes, means whereby the difierence inpotential between the junction of the capacitors and the centre tap isapplied to the input of an amplifier, a half wave rectifier whereby thesignal at the output of the amplifier is converted into a negativeunidirectional potential, and filter means whereby the negativeunidirectional potential is applied to the grid of the control tube soas to regulate the plate current therein.

9. The circuit as claimed in claim 8 in which the diffcrence inpotential between the junction and the centre tap is applied to theinput of the amplifier through a base clipper circuit whereby only agiven position of the negative peaks of the potential appears in theoutput of the amplifier.

References Cited in the file of this patent UNITED STATES PATENTS1,968,875 Cooper Aug. 7, 1934 2,052,413 Lord Aug. 25, 1936 2,310,112Palmer Feb. 2, 1943 2,315,619 Hutcheson Apr. 6, 1943

